Method of insuring the synchronization in television transmitters



July 8, 1941. K. SCHLESINGER 2,248,555

METHOD OF. INSURING THE SYNCHRONIZATION IN TELEVI SION TRANSMITTERS Filed March 8, 1938 F Patented July 8, 1941 METHOD OF INSURI'NG THE SYNCHRONIZA- TION IN TELEVISION TRAN SMITTERS Kurt'Schlesinger, Berlin, Germany, assignor, by

mesne assignments, to Loewe Radio, Inc., a corporation of New York Application March s, 1938, Serial No. 194,553 In Germany March 11, 1937 4 Claims.

In the usual method of synchronizinga wireless television transmission by the same frequency but higher amplitude there exists a disturbing influence of the image signals on the synchronization signals. Dependent on the fact as to whether the image at the moment of initiation of the synchronization signal, i. e. at the end of a line, is black, grey or white the time of the line change on the receiver varies by noticeable amounts. The impression is accordingly such that the contents of the image displace the synchronizationin the vicinity of the edge. In the case of moving pictures this effect is particularly disturbing if the contents of the image vary rapidly at the edge, as straight vertical lines of the image are rendered with continuously zation signals having a very steep front are very difficult to be generated, transmitted and dealtwith. 'It is accordingly necessary to find a method which is capable of dealingalso with flat synchronization signals the front of which embraces one or more image points.

The object of my invention are methods and means for obtaining a sharp synchronization in spite of synchronization signals having a fiat front.

Further objects of the invention may be best understood by reference to the following description taken in connection with the accompanying drawing in which- Figs. 1a to 1d are diagrams showing the disturbing effect of flat synchronization signals,

Fig. 2 shows an optical means for overcoming this effect whilst Figs. 3a to 30 are diagrams for explaining an electrical method for the same purpose.

According to the invention, a short period of waiting is introduced prior to initiation of the synchronization signal. During this waiting period the aerial current of the transmitter, by means which will be referred to later, is adjusted to an always constant waiting value independently of the contents of the corresponding edge of the image. In consequence the synchronization signals are always built up on the same absolute value of the condition of modulation in the transmitter. Accordingly, they do not require to be produced differently dependent on the top load or no-load condition, in which case it could not be avoided that exactly on account of their flat steepness of slope they reach the increased amplitude necessary for the synchronization in different periods of time.

Fig. 1 is intended to explain somewhat more concisely the conditions leading up to the described fault. The ordinate is, for example, the aerial current of the transmitter, the abscissa is the time t. In Fig. 1a there is shown a synchronization signal, the front of which requires a period of ascent 1- before it reaches its full value a. It is assumed that this period 1 is greater than an image point period, and accordingly it represents a lack of sharpness in the initiation of the signal. In Figs. lb, 10 and 1d there are shown three different conditions of control of the transmitter, in Fig. 1b the transmitter being at a dark image value, for example at black, at the moment h when the signal commences. 25% of the maximum value of the aerial current is shown. In Figs. 10 to 1d the corresponding momentary value of the transmitter at the edge of the image is at and respectively.

There is shown in the three figures the manner in which the same impulse signal in Fig. 1a, idealized by a straight ascending head. with a medium steepness sm tang. a, is disposed on these momentary values of the transmitter modulation. It is then to be recognized from the figures that the value of is attained by these signals at three entirely different points of time, viz., Tb, To and Td. It is assumed that this value represents the synchronization threshold in the receiver.

According to Fig. 1 (applying to so-called negative modulation) the synchronization threshold is reached most quickly in the case 1d, and most slowly in the case 1b, in which the transmitter was at black. The difference 6 in time between Tb and Td directly represents the lack of reliability in respect of the initiation of the next line and is equivalent to lateral variation in the position of image points which in superimposed lines should be situated one immediately below the other.

The displacement of the synchronization signals is accordingly found'to be the amount of the sub-tangent below a signal ascending with fiat steepness Sm in the case of a prescribed amplitude threshold as value of response of the synchronization.

This representation is not limited to a certain method of transmission, but occurs in the case of positive or negative modulation of the trans It will be clear from Fig-s. 1a to 1d that the effect disappears when the transmitter is always in the same condition of modulation upon the initiation of the synchronization signal. This can be accomplished according to Fig. 2. The image area is framed by the edges a, b. The

scanning image point passes over a larger area, viz., that framed by the edges and d. If the image change impulse occurred in the strip 2 and the line change impulse in the strip 3, these signals would immediately adjoin the image modulation at the edges 0. and b. According to the invention there is introduced a covering up by means of a special diaphragm 4, 5, i. e. an additional trimming down of the image area by the stated marginal portions. Contents of the image are accordingly sacrificed in this strip which is, however, very small. This way it is accomplished that the transmitter just before the transmission of the synchronization signal passes into a waiting position in the strips 2 and 3, which in the present case corresponds to an absolutely constant black. The duration of this period of Waiting, as regards the line impulse, must be greater than the maximum inertia of the transmission system, i. e. greater than the lack of sharpness of the received image (several image points). In practice operations have been successfully carried out with 4% waiting period and 6% line impulse period, so that altogether 10% of the line period was taken up by the synchronization.

The arrangement of a waiting strip 4 for the image change impulse is, generally speaking, unnecessary, as in the .case of normal simple scanning methods inertia considerations in respect of the image change do not enter into the question. It is only in the case of the interlaced line method that a waiting strip of this nature is also of value for the image change, in order to prevent the line jump from being rendered unsymmetrical. Unfortunately this method employing an odd number of lines is very sensitive to this fault.

It will be possible for every skilled man to apply to the transmitter, instead of the optical method illustrated in Fig. 2, a purely electrical one, imparting to the synchronization signals the steplike form shown in Fig. 3. In Fig. 3a, there is shown the form of the time curve of the signals in ideal form, in Fig. 3b with the unavoidable flattening, and in 30 the appertaining relative position of the modulation characteristic of the transmitter. The transmitter is controlled down to zero only in the case of full signal amplitude, which is designated 0.3. The waiting condition is set by as. The waiting period is inserted before the synchronizing signal by changing the last part of the video signal at the end of each line period respectively even of each frame period into a signal of a constant level. In order to malice this synchronization signal transmission electrically independent of the image signal transmission, the same, in accordance with the invention, is carried out one'radiofrequency stage before the image modulating stage. The carrier wave preliminary controlled by the synchronization signal 3 acts as potential for the next image modulating stage, to the grid of which it is coupled. Naturally the waiting period may also be made to be adjustable by shifting the knife edges 4 and 5 in Fig. 2.

I claim:

1. A method of transmitting television signals wherein picture and synchronizing signals follow each other in predetermined sequence and wherein the synchronizing signals are of a polarity in the direction of black in the picture'and of an amplitude greater than a maxium black signal, which comprises scanning an image area in a line-forline manner for developing image signals representing the contrast of a scanned picture area, "developing synchronizing signals each of a time duration at least equal to that of one image point of the scanned area, said synchronizing signals following each scanning line. and adjusting the picture signal voltage to a constant and predetermined level less than that of the synchronizing signals for an interval immediately prior to the initiation of a synchronizing signal impulse.

2. A method of transmitting television signals wherein picture and synchronizing signals follow each other in predetermined sequence and wherein the synchronizing signals are of a polarity in the direction of black in the picture and of an amplitude greater than a maximum black signal, which comprises scanning an image area ina line-for-line manner for developing image signals representing the contrast of a scanned picture area, developing synchronizing signals each of a time duration at least equal to that of one image point of the scanned area, said synchronizing signals following each scanning line, and optically developing a constant intensity signal of a predetermined level less than that corresponding to the amplitude of the synchronizing signal during a short interval immediately prior to the initiation of the synchronizing signal impulse.

3. A method of transmitting television signals wherein picture and synchronizing signa s follow each other in predetermined sequen. 2 and wherein the synchronizing signals are of a polarity in the direction of black in the picture and of anamplitude greater than a maximum black signal, which comprises scanning an image area in a line-for-line manner for developing image signals representing the contrast of a scanned picture area, developing synchronizing signals each of a time duration at least equal to that of one image point of the scanned area, said synchronizing signals following each scanning line, and electrically shaping the synchronizing signal impulse into a wave of step-like character with the first portion of the step having an amplitude less than that of the synchronizing signal proper and an intensity less than that necessary to initiate a synchronizing control at the receiver.

4. A method of transmitting television signals wherein picture signals are repeated in a linefor-line sequence and a line synchronizing signal follows each line of picture signals and a frame synchronizing signal follows a predetermined number of lines of picture signals and line synchronizing impulses and wherein the synchronizing signals are all of a polarity in the direction of black in a picture and of an amplitude greater than a maximum black signal, which comprises scanning an image area in a line-for-line manner for developing image signal representing the contrast of a scanned picture area, developing synchronizing signals at the end of each scanning line, said synchro-' nizing signals each being or a time duration at least equal to that of one image point in the scanned area, and each of said frame synchronizing signals being of a time duration greater than that of any line synchronizing impulses and electrically controlling the picture signal voltage during a short time interval immediately prior to the initiation of each synchronizing impulse to maintain the voltage level at a constant and predetermined value less than that of the synchronizing signals.

KURT SCHLESINGER. 

